1. Field
The present disclosure is directed to satellite positioning systems. More particularly, the present disclosure is directed to satellite positioning system time resolution.
2. Introduction
Presently, in a Global Positioning Systems (GPS), it is possible to obtain a GPS receiver location based on two pieces of information. The first piece of information is the relative propagation delay time of signals coming from different satellites, which is also referred to as pseudo-ranges. The propagation delay from at least four satellites can be necessary for a three-dimensional fix. The second piece of information is the location of each satellite when its signal was transmitted. Once this information is available it is possible to solve a location equation system. The location equation has four variables: three for the position of the receiver (xu, yu, zu) and a fourth for the time offset (Tu) between the receiver clock and the GPS system clock. Each satellite location (xi, yi, zi) is determined using ephemeris information, which is a refined model of the satellite orbit. This information can be obtained either directly from the satellites by data decoding or from other sources, like from a GPS assistance server.
Once the ephemeris information is available, the only missing piece of information is time. The absolute time in the GPS time system when the signal was transmitted by the satellite must be known in order to calculate the position. The time information can be obtained by decoding Time Of Week (TOW) information from the satellite in a standalone GPS receiver with a strong radio-frequency (RF) signal coming from the satellites. This information indicates the absolute GPS time of the beginning of the next GPS subframe. Each subframe lasts 6 seconds and a special sequence of bits called a preamble can be used to determine its beginning. If pseudo-range measurements are taken exactly at the beginning of each subframe for each satellite, the TOW information from the previous subframe can be used to calculate the position of each satellite at transmission time. In practice, it is not necessary to use the instant of the beginning of each subframe to take the pseudo-range measurement. In reality, if the location of the signal is known within a given subframe, the time offset can be compensated between that location in time and the beginning of the subframe.
Thus, it can be necessary to determine the relative location of a given instant in time of the received signal within the subframe. This instant in time will be used in the pseudo-range equations. An error in this time extraction process may have serious consequences in the final calculated position accuracy for two different reasons. The first reason is related to the fact that the satellites are moving at a speed of about 4 km/s. A time misalignment of 1 ms common to all satellites being used will result in a satellite location error of about 4 m, which will result in errors of the same order of magnitude in the receiver position calculation. The second reason is much more serious. If one satellite has a time extraction error of just 1 μs relative to the other satellites, this will correspond to an error of 300 meters for the range of that particular satellite, which will cause an error in the same order of magnitude in the final calculated receiver position.
The GPS L1 signal consists of an RF carrier (1.57542 GHz) bi-phased modulated by a 1023-chip long Pseudo-random Noise (PN) sequence. It is a Code Division Multiple Access (CDMA) system, where each satellite has its own unique PN sequence, which works as an identifier and allows for the separation of signals from different satellites. The chip rate is 1.023 Mcps (mega chips per second), and consequently, the transmission of one entire sequence takes 1 ms. Every 20 PN cycles (20 ms), the phase may or may not be inverted, which constitutes a 50 bps data modulation. 30 Data bits constitute one word (600 ms), and 10 words constitute one subframe (6 s). Five subframes constitute a page (30 s). The first 3 subframes carry information needed for position calculation, while the last 2 subframes carry a set of other parameters that are transmitted over 25 pages commuted over 12.5 minutes. Typically, a GPS receiver will have correlators that will try to align an internally generated PN sequence with the incoming signal. Finding a peak in the correlation gives a good measurement of time alignment within the 1 ms duration of the PN sequence. If the signal is strong enough so that the data can be properly demodulated, and if the data capture window is long enough, then the TOW can be obtained and the exact location of any given 1 ms-long PN cycle within the subframe can be determined.
In other situations, due to unfavorable RF conditions, the signal may not be strong enough for bit-by-bit data demodulation, but it may be strong enough so that the peak in the PN correlation can still be determined. This may be possible due to the use of longer integration times. In a typical receiver, this happens when the incoming signal strength for each satellite is below −142 dBm and still above a certain threshold which could typically be in the −155 dBm range. In other cases, the data acquisition time window may be small, not allowing for the correct determination of the location of a particular short sequence of bits within the GPS data stream.
In these cases, part of the time information can still be found within the 1 ms time window of the PN sequence due to chip alignment, which is also referred as code phase information. However, the position of that particular PN sequence is not known within the GPS data stream. In the case of a 6 second-long subframe there are 6,000 possibilities of alignment. This is like having a broken watch with only the minute pointer, but not the hour pointer. In the context of this disclosure, this is what is being called the time ambiguity problem in GPS.
In Assisted-GPS (A-GPS) systems, when coarse time aiding information is provided, which for illustration purposes can be assumed as +/−2 seconds accurate—the search space in the time domain becomes restricted to a 4-second wide window. This window may be positioned within one or two 6-second long subframes. The position of the signal within the 1 ms duration of the PN sequence can be obtained from the correlation process. However, there is still the problem of aligning a given PN sequence beginning within the subframe in the 4-second windows, which gives 4,000 possibilities. There are known methods that can be used to attempt to solve this ambiguity, like the one proposed by Mike King in the U.S. Pat. No. 6,346,911, but they may require some significant amount of processing, given the high number of time alignment possibilities.
Thus, there is a need for a method for reducing the number of possible time alignment possibilities to simplify the satellite positioning system time-ambiguity resolution problem.